The present invention relates to the disposal of solid materials by incineration, and in particular the disposal of solid materials which generate highly pollutant gases when they burn.
This type of material is very wide-ranging and comprises a large variety of synthetic products, plastic materials, and others.
This type of material is usually disposed of by incinerators, which involve a considerable energy cost and, notwithstanding gas purification, present drawbacks concerning the production of gaseous dioxins and other pollutant gases.
In addition to an understandable plant complication, the known processes involve oxidizing the treated material, so depleting its calorific value and rendering it very deficient as a fuel.
It is immediately evident that this considerably affects the economy of the process. The object of this patent is to dispose of materials consisting in particular of plastic materials of synthetic origin by the destruction or depolymerization of their CH chains, with simultaneous recovery of the resultant liquid condensate of the process, which has high energy power.
A last but not less important object of the invention is to implement cleaning of the off-gases such as to satisfy the current severe ecological regulations.
These objects of the invention are attained by the process and plant defined in the claims.
The process of the invention represents the combination of two mutually interacting steps, and takes place under vacuum at low temperature in the presence of an oxidizing agent such as oxygen or ozone.
The material to be treated is preferably comminuted into pieces of uniform dimensions, of the order of 50 mm to 300 mm, and then fed into a vessel acting as a reactor.
The quantity of material, or charge, fed into the vessel preferably occupies a quarter of its capacity, below which a normal burner, of the type used in common boilers, opens into the vessel.
The system is initiated by flame triggering with said burner.
The time required for triggering by the flame is of the order of one minute.
When triggering has occurred, the entire vessel or reactor is subjected to forced suction by a fan by which it is emptied of its initial reaction gas, the suction fan after a short period then reducing the pressure within the vessel or reactor to prevent undesirable combustion reactions.
The absolute pressure within the reactor is stabilized at around 250 mm Hg.
The lack of air within the reactor considerably slows down the combustion process, which was initially accompanied by flame, to then become increasingly more similar to thermal cracking.
After this initiation step, with the reaction stabilized, normal operation commences, during which an oxidizing agent, generally oxygen, is fed into the reaction chamber in a quantity from 0.5 to 1.5 wt % of the material to be treated.
Care must be taken to feed the oxidizing agent, facilitating the process of molecular breakdown, at a rate which does not result in the creation of centres of combustion with triggering of flame.
During the entire reaction, the feed of oxidizing agent is constantly balanced with the quantity of reaction gas generated by the molecular splitting of the polymer chains, in order to favour the entire process to the maximum extent.
The molecular splitting of the polymer chains is aimed at restoring certain chemical parameters which preceded the formation of polymer chains, in the absence of combustion and with the intention of obtaining the greatest possible gas quantity as the final result of the reaction.
The reaction gases obtained in this manner are fed to a condenser which converts them into liquid form (phase).
The process comprises purifying the obtained liquid phase of undesirable substances and by-products, principally the sulphur component if present in the starting materials. Another undesirable component is the chlorine component which, if present on entry, is eliminated by circulation through a suitable removal circuit followed by neutralization by treatment with calcium carbonate CaCO3 or sodium bicarbonate 2NaHCO3.
The liquid cleaned of the undesirable components is fed to certain vessels performing the double task of storage and further purification of the product by gravity.
The entire system requires a suitable vent to prevent pressurization during the cycle. In this manner there is extracted from most of the polymerized CH chains a quantity of gas and/or liquid of high calorific value and at low depolymerization cost.
All those parts which cannot be transformed are discharged at the end of the cycle and are classed as process residues.
Said residues are not necessarily unsuitable for other possible uses, but are unsuitable for adequate transformation with the present process.
The nature of the residues in question means that they may be able to be reused by other techniques of destructive type such as combustion, or of selective type based on individual polymer chains, with consequent processing to obtain products suitable for moulding or extrusion.
If required by local regulations, the process can include cleaning of the off-gases and emissions by known means.